This invention relates to a directional drilling system for use in the formation of boreholes, for example for subsequent use in the extraction of hydrocarbons.
A number of directional drilling systems are known. For example it is known to provide a drilling system with a bias unit operable to apply a laterally directed force to a drill bit thereof to urge the drill bit towards a position in which it is offset from the axis of the borehole so that continued operation of the drilling system results in a deviation being formed in the borehole. Another form of steerable drilling system incorporates a bent housing or sub, the orientation of which can be adjusted to control the direction in which the associated drill bit is pointed so as to control the direction in which continued operation of the drilling system extends the borehole.
WO03/008754 describes an arrangement in which a pilot bit is located ahead of a main bit, the pilot bit being mounted upon a spherical bearing to allow it to be pointed in a desired direction and thereby achieve steering of the drilling system.
It is an object of the invention to provide a directional drilling system of relatively simple and convenient form.